OK - back from Texas and all the parts arrived while I was gone. Populated one node with L5638s and 48GB RDIMM and lit it up.
The PSU fans are surprisingly quiet for older 1U redundant 1100watt PSUs. Not near silent, but not obnoxious at all. I don't think I'll need to do anything with those.
When the unit starts up it ramps all the fans to full speed for a few seconds. WOW - its pretty loud! But then they settle down to a much slower speed which is much quieter - according to the sensors in the BMC they are sitting at about 4,000RPM. In fact, compared to the Juniper 10Gbe switch its not too bad at all. Louder than the Juniper, but the tonality of the 80mm fans is much less obnoxious than the 40mm fans in the Juniper EX. If they never ramped up I might just be willing to leave them alone for my garage-based lab - but I know under load they will come up.
After thinking about it while I was traveling I think I'm just going to go straight to the fan replacement. I bought the Evercool's and connectors to do a dell 6-pin PCM to standard 4-pin PCM. Need put together the other three nodes and load an OS so that I can get temp readings to know that the Evercool's move enough air. Should have results sometime late this weekend.